And in fact, protecting your non-progressive incumbents is likely more efficient in the long run precisely because their incumbency lets them win without using up quite as much central-allocated resources (in part because they are able to generate more local resources as incumbents, thereby increasing the total size of the pie available for allocation).

That is because incumbency really does make a difference in electoral odds, so you should be careful about reallocating too much funding away from non-progressive incumbents: ultimately you are likely going to need some non-progressives to make up a majority, and those incumbents are your best bet.

Incidentally, the same is true in the House: the non-progressive Representatives are less than a majority of the caucus, but the progressive Representatives are less than a majority of the House as a whole.